DRBD with external meta disk

I am attempting to create a failover with DRBD and I have two partitions.

Partition 1) /dev/sda4 is setup for the KVMs I will be creating. Partition 2) /dev/sda5 is for the DRBD metadata

My config file is below

resource r0 {
    protocol C;
    startup {
            wfc-timeout  15;
            degr-wfc-timeout 60;
    }
    net {
            cram-hmac-alg sha1;
            shared-secret "SECRET_KEY";
    }
    on Server1{
            device /dev/drbd0;
            disk /dev/sda4;
            address IP:7788;
            meta-disk /dev/sda5;
    }
    on Server2{
            device /dev/drbd0;
            disk /dev/sda4;
            address IP:7788;
            meta-disk /dev/sda5;
    }

}

When I run drbdadm create-md r0 it runs successfully but it creates /dev/drbd0 on both partitions.

    ├─sda4      8:4    0  7.2T  0 part
    │ └─drbd0 147:0    0  7.2T  1 disk
    └─sda5      8:5    0  399M  0 part
      └─drbd0 147:0    0  7.2T  1 disk

It's my understanding that it should only create it on the meta disk which is /dev/sda5. The reason I setup the meta disk is to avoid any writing to sda4. Am I correct in that reasoning or am I missing something.

I confirmed this is how lsblk output looks on AlmaLinux 8.5 when using external meta-data with DRBD. The DRBD device is only a single virtual block device layered over both partitions in your setup.

It works as expected. If you inspect the block devices using other methods/utilities you'll see that you only have a single /dev/drbd0, and the metadata partition you created is the size you'd expect.

For example, my configuration using two LVM volumes:

resource r0 {
    protocol C;
    startup {
            wfc-timeout  15;
            degr-wfc-timeout 60;
    }
    net {
            cram-hmac-alg sha1;
            shared-secret "SECRET_KEY";
    }
    on Server1{
            device /dev/drbd0;
            disk /dev/drbdpool/data;      # 1GiB lvm on /dev/sdb
            meta-disk /dev/drbdpool/meta; # 4MiB lvm on /dev/sdb
            192.168.1.11:7788;
    }
    on Server2{
            device /dev/drbd0;
            disk /dev/drbdpool/data;      # 1GiB lvm on /dev/sdb
            meta-disk /dev/drbdpool/meta; # 4MiB lvm on /dev/sdb
            192.168.1.12:7788;
    }

Shows exactly what I'd expect in /proc/partitions:

# cat /proc/partitions 
major minor  #blocks  name

   8        0   20480000 sda
   8        1    2048000 sda1
   8        2   18430976 sda2
   8       16    8388608 sdb
 253        0       4096 dm-0
 253        1    1048576 dm-1
 147        0    1048576 drbd0

blockdev --report looks good too:

# blockdev --report /dev/drbd0
RO    RA   SSZ   BSZ   StartSec            Size   Device
rw   256   512  4096          0      1073741824   /dev/drbd0
# blockdev --report /dev/drbdpool/meta 
RO    RA   SSZ   BSZ   StartSec            Size   Device
rw  8192   512  4096          0         4194304   /dev/drbdpool/meta
# blockdev --report /dev/drbdpool/data
RO    RA   SSZ   BSZ   StartSec            Size   Device
rw  8192   512  4096          0      1073741824   /dev/drbdpool/data

This is likely where lsblk is getting confused:

# cat /sys/block/drbd0/size 
2097152
# cat /sys/block/dm-0/size 
8192
# cat /sys/block/dm-1/size 
2097152
# cat /sys/block/dm-0/holders/drbd0/size 
2097152
# cat /sys/block/dm-1/holders/drbd0/size 
2097152